Poly(1,4-bitylene terephthalate) is a known linear polyester molding resin that -butylene useful for many diverse applications. This material is described in Whinfield. U.S. Pat. No. 2,465,319 and Pengilly, U.S. Pat. No. 3,047,539 which are hereby incorporated by reference.
Poly(1,4-butylene terephthalate) is commercially prepared by reaction of a dialkylterephthalate or terephthalic acid with 1,4-butanediol at elevated temperature and reduced pressure.
The apparatus employed in preparing poly(1,4-butylene terephthalate) is subject to unexpected mechanical failure that results in holding partially polymerized, in process polymer, above its melt temperature until the mechanical difficulty has been overcome. In some cases, the low molecular weight polymer must be removed prematurely from the system.
Poly(1,4-butylene terephthalate) has been found to be highly sensitive to interruption in the normal polymerization process. These interruptions result in a low molecular weight poly(1,4-butylene terephthalate) which has a tendency to undergo thermal degradation. The thermal degradation is evidenced by the sublimation of volatile materials from the melt. These volatiles tend to condense in the vacuum system and cause fouling of the equipment and eventually further interruptions in the polymerization process.
The thermal degradation of poly(1,4-butylene terephthalate) is believed to result in the splitting off of the hydroxyl terminal group (--O--CH.sub.2 --CH.sub.2 --CH.sub.2 --CH.sub.2 --OH) with the formation of a terminal carboxylic acid group and tetrahydrofuran. Other low molecular weight polyalkylene terephthalates do not exhibit this phenomenon as ethylene glycol and propylene glycol do not readily cyclize for steric reasons. When poly(1,4-butylene terephthalate) undergoes thermal degradation, an excess of carboxylic acid groups is rapidly formed. These carboxylic acid groups act as chain stoppers and prevent further build up of molecular weight upon resumption of the process.
It has now been found that the addition of a minor amount of a compound containing at least two aliphatic or cycloaliphatic hydroxyl groups prevents thermal degradation of the low molecular weight poly(1,4-butylene terephthalate) and permits it to be polymerized in the melt.
Accordingly, it is a primary object of this invention to provide a novel process for the melt polymerization of low molecular weight poly(1,4-butylene terephthalate) to high molecular weight poly(1,4-butylene terephthalate).
It is also an object of this invention to provide a method for preventing the thermal degradation of low molecular weight poly(1,4-butylene terephthalate).